One of the most critical aspects of the control of railway vehicles, particularly freight trains, is the predictable successful operation of the air brake system, which is subjected to a variety of dynamic effects as a result of the controlled application and release of brake pipe pressure in response to varying conditions encountered by the train.
In a typical air brake system employed by a railway freight train, the application and release of braking action is controlled by the engineman through an air brake control system in the locomotive. In such systems, braking force is conventionally applied to the wheels of each train car by pneumatically operated braking mechanisms in response to a reduction of the air pressure communicated to the braking mechanisms through a train air line which is coupled to a pressurized brake pipe in the air brake control system. Upon completion of a braking operation, the braking force applied by the respective braking mechanisms is released in response to restoration of the pressure in the train air line by the air brake control system operated by the engineman.
In braking systems of the type described, the magnitude of the braking force applied to the wheels of the train cars by the pneumatic braking mechanisms is directly proportional to the magnitude of the differential pressure reduction in the air line sensed by the respective braking mechanisms. However, due to the substantial volume of the air line, which extends the entire length of the train, an appreciable amount of time is required for the restoration of pressure in the air line to its original value following a braking operation. A subsequent application of the brakes prior to complete restoration of pressure in the air line thus results in the communication of a smaller differential pressure reduction to the pneumatic braking mechanisms at the train car wheels, and therefore less braking force than requested by the engineman via the air brake control system.
Even when he recognizes the insufficiency of the new braking application, the engineman often attempts to remedy the problem by a further application of the brakes. Again, however, the application of a braking effort which is less than expected will take place, so that there may still be inadequate braking action applied by the cars of the train. Simply put, if the engineman tries to make up for insufficient braking of one pressure reduction request in a piecemeal fashion, and under-corrects each time, it is possible that continuing efforts in this process will be unsuccessful due to increasing train speed, and that the originally intended braking effort and train speed will never be accomplished.
A method and apparatus to remedy this situation are disclosed in our commonly assigned U.S. Pat. No. 4,859,000, in which the air brake control system is modified to augment automatically the pressure reduction effected by the engineman whenever the brakes are applied prior to complete recharging of the train air line, so that the total amount of the pressure reduction communicated to the pneumatic braking mechanisms closely approximates that which would have been achieved had the train air line been fully charged. The improved adaptive braking system according to the above mentioned patent therefore increases the accuracy of the adaptive braking function by taking into account, and compensating for, first order transient effects in the charging and discharging of the train air line.
We have discovered that further improvements can be made to our adaptive braking system in order to provide a total braking effort which more closely approximates that expected by the engineman. In particular, the sensing method described in our U.S. Pat. No. 4,859,000 measures the decrease in the air line charging flow rate to signal the "equivalent pressure" of the train air line. This sensing method is characterized by inaccuracies due to hysteresis and frictional effects in the brake system control mechanism. In addition, the presence of second order transient effects in the propagation of pressure waves over the length of the air line during charging and discharging (often referred to as a "pressure hill"), provides a further source of error in the correction applied by the control system.
Accordingly, it is an object of the present invention to provide an improved adaptive air braking system which is not subject to the above mentioned hysteresis and frictional effects, and further compensates for inaccuracies in the system by taking into account the pressure hill effect.
This and other objects of the present invention are achieved by directly sensing the pressure in the brake line of the brake control system using a pressure transducer mounted thereon, and by providing a set of look up tables containing experimentally determined pressure reduction factors to compensate for the pressure hill and related effects. A central signal processing unit is programmed to select a second order correction value from the look up tables based on the magnitude of the previous reduction requested by the engineman. The correction value thus determined is then decremented over time so as to decay to zero when all transient effect in the brake control system have been eliminated. In the event that the brakes are reapplied prior to complete recharging of the train air line following a braking operation, an additional pressure reduction equal to the decremented correction value is added to the pressure reduction effected by the engineman and the first order correction referred to above.
Other objects, advantages and novel features of the present invention will be apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.